A spectrophotometer is usually the most accurate and versatile of the devices used for measuring or monitoring color of an object based upon a dispersion of light emitted or reflected from the object and dispersed into a plurality of separate wavelengths. A colorimeter belongs to a subclass of spectrophotometers that is concerned primarily or solely with the strengths of the different wavelengths when compared with a CIE standard for color such as 1931 CIE XYZ. Standard colorimetry determines tristimulus values received by a color discrimination module with reference to certain standard values such as the widely accepted 1931 CIE XYZ standards. Colorimetry may be used to assess the colors produced on a video screen, in paints, at solid surfaces such as paper and plastics, as transmitted by or reflected from liquids, in dyed fabrics and textiles, in natural materials such as leather and wool, and as produced by computer peripheral devices such as color monitors, color printers and color scanners. Ideally, a colorimeter will perform accurate color measurements for any self-luminous body and for any light issued from a body illuminated by a separate light source.
The invention disclosed herein is applicable to spectrophotometers generally and more particularly to colorimeters. For convenient reference, the apparatus disclosed herein will be referred to as a colorimeter, without intending to limit the scope of application of the apparatus.
Hambleton, in U.S. Pat. No. 3,490,849, discloses a colorimeter in which light from a sample is diffused in a sphere and passed through a filter to a photomultiplier tube. A plurality of filters must be inserted, one at a time, in order to adequately cover the spectral range of interest.
U.S. Pat. No. 3,525,572, issued to Hunter et al., discloses reflection of light from the surface of a fabric or similar light-reflecting object, and passage of this light to a sequence of photomultiplier tubes covered with tristimulus color filters. The tube signals are passed through certain circuits for analysis and determination of the color of the light received by the tubes. Two or more light sources may be used to separately illuminate the sample for purposes of color determination.
U.S. Pat. No. 3,880,523, issued to Thomas, discloses a multi-channel colorimeter in which a diffraction grating is used to decompose an incoming light beam into a plurality of non-overlapping spectral bands, with each such band being directed along a separate light waveguide for use in colorimetry.
Suga discloses use of three filter mirrors, each corresponding to a different color, as part of a colorimeter in U.S. Pat. No. 4,150,898. Light produced by the sample is separately reflected from each filter mirror and is received as a first light component by a phototube, which also receives a second light component directly from a light source used to illuminate the sample. The first and second light components are compared to determine the composition of light from the sample that is reflected by the particular filter mirror. Three or more filter mirrors are used, and each filter must be positioned separately to reflect light to the phototube. Simultaneous filtering at all wavelengths of light from the sample is not available.
A trichromatic colorimeter for obtaining CIE chromaticity coordinates, disclosed by Yuasa in U.S. Pat. No. 4,402,611, separates a light beam into three components and passes each beam component through a separate color filter corresponding to one of the tristimulus spectra. The three filtered light beams are then passed to separate photosensor arrays to produce electronic output signals that may be analyzed to determine the spectrum of the incoming light beam.
Kurandt, in U.S. Pat. No. 4,838,697, discloses use of two or three dichroic mirrors that receive light and filter it in different portions of the visible spectrum. The filtered light is then analyzed colorimetrically.
Steenhoek, in U.S. Pat. No. 4,917,495, discloses use of 12 photodetector elements that decompose an incoming light beam into separate spectral bands, for subsequent color analysis by computation of the tristimulus integrals for the incoming light beam.
In using a spectrophotometer, it is usually necessary to disperse the incident light beam into a plurality of different wavelengths or wavelength intervals in order to fully analyze the content of the incident light beam. Wavelength dispersion, using a plurality of beamsplitters, each functioning in a different part of the wavelength spectrum, is disclosed by Perkins in U.S. Pat. No. 4,681,445.
Crane, in U.S. Pat. No. 4,743,114, discloses the use of Fabry-Perot interferometer scanning using a nutating etalon in which the incidence angle of the light beam relative to the interferometer is caused to vary periodically in two perpendicular directions of rotation.
Use of a blazed diffraction grating or similar means of wavelength dispersion is disclosed in U.S. Pat. No. 4,758,090, issued to Schuma, in U.S. Pat. No. 4,718,764, issued to Fink, and in U.S. Pat. No. 4,776,696, issued to Hettrick et al.
Rodine, in U.S. Pat. No. 2,960,015, discloses a method of making variable transmission light filters in a two-dimensional, radially symmetric configuration in which filter transmissivity varies with radial distance from the center of a circular pattern.
A variable color filter is disclosed by Illsley et al. in U.S. Pat. No. 3,442,572, using a wavelength filter positioned on the circumference of a large circle, where the filter thickness increases linearly with increase of the azimuthal angle .theta.(0.ltoreq.8.theta.&lt;2.pi.) of the position on the circle circumference. This method relies on a fabrication method and apparatus, disclosed and claimed in two division patents, U.S. Pat. Nos. 3,530,824 and 3,617,331, but probably cannot be used to fabricate a filter whose thickness is not linearly increasing with increase in a spatial coordinate.
Fein et al. disclose an optical radiation translating device in U.S. Pat. No. 3,498,693. The Fein et al. apparatus in one embodiment (FIG. 3) uses two spaced apart planar reflectors of light that are inclined at a non-zero angle relative to one another, with a wedge-shaped dielectric material occupying the volume between the two reflectors. Light is transmitted through the wedge-shaped filter, requiring constructive interference of the light waves, only at positions along the device where the one way optical path length of the light beam through the dielectric material is an integral multiple of one half of the wavelength .lambda..sub.0 of the light. The light is assumed to be monochromatic.
In U.S. Pat. No. 3,552,826, Hanes et al. disclose a variable thickness, multi-layer light reflector with a thickness h(x) that decreases exponentially with increase in a spatial coordinate x, measured in a predetermined direction in a plane of the reflector. The reflectance R of the reflector at any point x is a function of the single variable w=.lambda./h(x), where .lambda. is the wavelength of light incident on the reflector. The exponential decrease of thickness h(x) with the coordinate x is required in order to ensure that .delta..sup.2 R/.delta..lambda..delta.x=0 and .delta..sup.2 R/.delta.x.sup.2 =0.
Bates, in U.S. Pat. No. 3,929,398, also discloses use of a wedge-shaped interference filter to produce a line of light at a particular coordinate position x that varies with the wavelength of the incident monochromatic light. The position x of the line of light is variable and is controlled by the operator's choice of wavelength of the incident light. A sequence of masks is used to selectively mask portions of the line to produce an ordered sequence of dark and light regions on the illuminated line that characterizes the light (e.g., each wavelength).
A color sensing device using a group of adjacent, non-overlapping light filters with different pass bands is disclosed by Hinoda et al. in U.S. Pat. No. 4,547,074. Each light filter consists of an interference filter with a plurality of separated wavelength pass bands plus a color filter with a sharp cutoff band that falls within one of the interference filter pass bands. The serial combination of these two filters selects a particular, fixed narrow wavelength band for transmission of light therethrough. A photodiode, positioned beneath the serially combined interference filter and color filter, receives the transmitted light and determines the relative intensities of light in each of several wavelength pass bands. Photodiode light-receiving faces may have different areas to reflect the light sensitivity of the photodiodes in different, fixed wavelength regions. A subgroup of such filters may be configured to sense the relative amount of light in each of a set of adjacent wavelength bands, to thereby produce color matching capability according to the CIE XYZ colorimetric system. The incident light is not assumed to be monochromatic, but it appears that each interference filter/color filter pair must be carefully matched to provide a fixed wavelength pass band.
Owen, in U.S. Pat. No. 4,054,389, discloses a spectrophotometer in which light is first collected by a bundle of optical fibers, each of which delivers light to a separate position on a light-receiving surface. The light-receiving surface may be a first surface of a wedge-shaped linear variable interference filter that has a second opposing surface at which an array of photosensors is positioned to receive the light passed through various sections of the interference filter. Each photosensor responds to light of a different wavelength delivered by one of the optical fibers.
U.S. Pat. No. 4,822,998, issued to Yokota et al., discloses use of an array of light sensors, each sensor being sensitive to a different wavelength range in receiving light transmitted through a light filter with a transmission wavelength band pass corresponding to the wavelength band to which the light sensor responds. In one embodiment, shown in FIG. 1 of the Yokota et al. patent, the light filter array is arranged in a double staircase configuration, with the filter thickness increasing from one plateau of constant thickness to another plateau of greater constant thickness. A first filter staircase and second filter staircase have filter thicknesses chosen to correspond to optical interference orders m=1 and m=2, respectively, according to well known optical relations for a Fabry-Perot etalon. By separating the visible spectrum (wavelengths .lambda.=0.4-0.7 .mu.m) into two smaller wavelength ranges, the sidebands of each interference order, other than the order m=1 or m=2 that is desired, are caused to appear at wavelengths well removed from the visible spectrum and can be attenuated with simple fixed band pass ultraviolet and/or infrared filters. Low order Fabry-Perot interference bands are usually not narrow enough by themselves for most spectrophotometer applications. As FIG. 7 of the Yokota patent illustrates, the full width at half maximum ("FWHM") for a low order interference band, with a central wavelength .lambda..sub.c =400 nm, is 15 nm and 9.6 nm for surface reflectivities of R=0.23 and 0.62, respectively. The FWHM will increase with increase in wavelength. These FWHM values are much too wide for many applications of such technology in colorimeters and radiometers. Increasing the reflectivity R of the surfaces of the Fabry-Perot etalon will narrow the FWHM by a modest amount, but the FWHM is still too large for some spectrophotometer applications, and the transmissivity T=1-R may already be so low that the signal-noise ratio for the photosensor signals becomes a concern The wavelength skirts that extend beyond the FWHM wavelength region may also be too broad to allow sharp wavelength discrimination.
A wedge-shaped filter spectrometer is disclosed by Pellicori et al in the patent application PCT/US88/03898, published circa 15 June 1989. The Pellicori et al. spectrometer provides a sequence of wedgeshaped order-wavelength thickness dielectric layers of alternating refractive indices high (H), low (L), high (H), low (L), . . . that are deposited on one another in accordance with a sequence H LL HLHLHL HH LHLHLH LL HL. The thickness of each of these layers has a constant slope as a spatial position coordinate x changes in a selected direction across the face of the filter Pellicori et al. note that undesirable sidebands, appearing at wavelengths removed from a central wavelength .lambda..sub.c, are present and suggest the use of an interference filter blocking stack, with an apparently fixed pass band, to prevent the appearance of the side bands in the transmitted light beam.
Monitor-specific calibrators are available which measure the brightness of a portion of a CRT screen under specific conditions. The sensor is usually a single silicon photodiode, and each CRT gun is turned on individually (e.g., red, then green, then blue) over a sequence of successively greater brightnesses to determine the gamma, offset and gain parameters for that color and that device. This information is fed into an associated computer to correct the CRT amplifiers in order to correct any errors present. The phosphor set used with the color monitor must be known exactly so that such a device is not universally applicable to CRTs of other manufacturers or even other products of the same manufacturer. In particular, the monitor-specific color calibrator is not useful for color printers, for general color calibration, or for use of ambient lighting rather than the specific lighting for which the color calibrator is set. Examples of such devices are light calibrators offered by Barco Industries for their color monitors, and by Radius, Inc. for the Apple Macintosh color monitors. These devices function as calibration probes that are electronically tethered to a host computer.
An example of a tristimulus colorimeter and calibration system is shown in U.S. Pat. No. 4,500,919, issued to Schreiber. This patent discloses use of a color scanner as a calibration device to produce a set of three tristimulus signals dependent on the colors sensed in the original document. The scanner measures red, green and blue components as usual and is used to calibrate the colors of print samples. The RGB signals are not colorimetrically correct, unless matched with the eye for specified lighting conditions, the scanner produces color errors. Here, a color scanner is used as a tristimulus color input device without comparison to the basic XYZ color standards. A scanner would be unable to determine the color transfer function of a color monitor, as required for a general purpose colorimeter.
Many of the devices of the prior art are large and bulky and do not make full use of or analyze all wavelengths in a continuous wavelength interval of the incident light beam. The cost of these devices is usually great, due in part to the delicate optical systems used. Further, no controllable means has been disclosed for compensating, at the same time, for the non-uniform sensitivity, as a function of wavelength, of photodetector elements or for compensating for use of a non-standard light source for illumination of an object whose color is to be measured or monitored. Finally, the devices in the prior art compensate for the presence of undesirable bands, if at all, only through use of a fixed pass band filter that is used in combination with the wavelength dispersion means.
Several other technology-based problems recur, including the problems that: (1) color errors arise because the filters cannot provide a perfect match of the XYZ tristimulus response of the human eye; (2) use of a set of fixed filters is applicable only to a fixed ambient lighting condition, with no flexibility in use of other available light sources; and (3) the XYZ tristimulus values sensed by the colorimeter must be read and manually entered into a computer for further computations.
Each peripheral device, such as a color monitor, color printer or color scanner, produces colors using intrinsically different colorants, such as phosphors used in monitors, color dyes and pigments used in printers used, and filters used in color scanners. The rendition of colors, given the same color input signals, thus varies with the device (device dependence). Some workers have attempted to electronically correct for intrinsic wavelength-dependent differences, such as lower sensitivity of photosensors at the blue end of the visible spectrum vis-a-vis the red end of the spectrum. However, color errors still persist because the color transfer functions of these devices are not entirely stable: amplifier drift occurs in a color monitor; printer colors change with the paper or substrate upon which the printing occurs; and color scanners also have drift problems. Further, the manufacturer cannot predict the ambient lighting conditions under which the computer system will operate. The net result is that the rendered images of the peripheral device may vary with time and may be incorrect over all. Data sent from one computer to another may not provide matching colors between the two systems.
A need also exists for a means to add "real" colors to CAD rendering software. The computer industry has gone to great lengths to add realism to CAD rendering programs such as ray tracing algorithms. To date, this greater realism has been realized only through the use of synthetic color palettes with computer-generated colors. It would be more appropriate to be able to input the color values of a real object that the CAD software is rendering.
What is needed is a compact color calibration apparatus that (1) efficiently disperses a light beam into a continuous interval of wavelengths and analyzes the content of the light beam throughout this interval; (2) allows shifting of augmentation of the wavelength interval to be analyzed; (3) allows flexibility and alteration of the light sources and light beam intensity distribution received by a plurality of wavelengthsensitive photodetector elements; (4) provides sharply defined, very narrow bands of light of different wavelengths at each photodetector element with no side band problems; (5) allows construction of the apparatus on a single chip that is compact and rugged and has low cost; (6) allows use of this apparatus for spectrophotometry or colorimetry work on a variety of self-luminous or nonself-luminous objects whose color is to be measured or monitored; and (7) allows automatic entry of the tristimulus values sensed into a computer for further analysis and computation.